1. Field of the Invention
The present invention relates to a developing device for developing a latent image formed on an image carrier with a developer and a printer, facsimile apparatus, copier or similar electrophotographic image forming apparatus using the same.
2. Description of the Background Art
One of conventional developing devices for use in electrophotographic image forming apparatuses is configured to cause a developer, which is a toner and magnetic grain mixture, to magnetically deposit on a developer carrier in the form of a magnet brush. The magnet brush is brought into contact with a latent image formed on an image carrier in movement, thereby developing the latent image with toner. This type of developing device is taught in, e.g., Japanese Patent Laid-Open Publication Nos. 5-40406, 7-72739, 8-137255, 10-20664, 10-232561, 11-38761, and 11-338259.
In the developing device of the type described, the developer carrier is made up of a sleeve and a magnet roller fixed in place inside the sleeve. The magnet roller is provided with a magnetic pole for exerting a magnetic force in the directions normal to and tangential to the sleeve, so that the developer is retained on and conveyed by the sleeve. A doctor or metering member faces the surface of the sleeve in order to regulate the amount of the developer deposited on the sleeve. The normal magnetic force mainly serves to retain the developer at the position of the magnetic pole while the tangential magnetic force mainly serves to convey the developer along the surface of the sleeve. A developer staying portion is formed upstream of the doctor in the direction of developer conveyance for promoting the frictional charging of the developer, so that the developer can rise on the sleeve in the form of a magnet brush or brush chains.
While various schemes have heretofore been proposed for implementing dot reproducibility high enough to insure high image quality, it is well known that reducing the grain size of the magnetic grain of the developer, among others, is effective.
To achieve high image quality. With an image forming apparatus using the developing device of the type described, it is necessary to stabilize the amount of the developer scooped up, to protect the developer from deterioration, and to make the amount of charge to deposit on the developer adequate. The amount of the developer scooped up refers to an amount in which the developer passes a gap between the sleeve and the doctor. The magnetic force distribution of the magnet roller, as measured on the surface of the sleeve facing the doctor, contributes a great deal to achievement of the above objects.
More specifically, if the tangential magnetic force on the sleeve surface facing the doctor is excessively small, then the conveying ability of the sleeve is noticeably effected by the wear of the sleeve surface and that of the developer surface, causing the amount of the developer scooped up to noticeably vary. Also, the normal magnetic force on the sleeve surface is excessively greater than the tangential magnetic force on the sleeve surface, increasing the amount of the developer to be retained on the sleeve. As a result, excessive friction acts between the grains of the developer and deteriorates the developer (so-called toner spent), preventing image quality from being enhanced despite the use of magnetic grains of small grain size.
On the other hand, if the tangential magnetic force of the magnetic pole facing the doctor is greater than the normal magnetic force of the same pole, then the tangential magnetic force improves the conveying ability of the sleeve for thereby reducing the variation of the amount of the developer scooped up ascribable to the surface configurations stated above. It has been proposed to cause the doctor to face the portion of the sleeve surface where the normal tangential force is smallest in order to enhance the conveying ability. This kind of scheme, however, reduces the amount of the developer to be retained on the sleeve and therefore friction to act between the developer grains, causing the background of an image to be contaminated and degrading the ability to follow a solid image. Particularly, a developing device for a color image forming apparatus tends to consume much toner because color images include more solid images than conventional monochromatic images. It is therefore difficult to deposit a desired amount of charge on a great amount of toner. A short amount of charge on toner is apt to bring about background contamination, fog and other image defects.
A magnetic member may be positioned on the metering member of the magnet brush type developing device, as also proposed in the past. The magnetic member causes the magnetic grains to chain in a doctor gap between it and the sleeve surface in the form of a curtain by exerting a restraining force, so that the toner grains passing through the curtain are additionally charged on contacting the magnetic grains. However, if the restraining force of the magnetic member is excessively great, then excessive stress acts on the magnetic grains and sleeve and is apt to cause their surfaces to wear. Consequently, frictional resistance on the surface of the sleeve and the surfaces of the magnetic grains is reduced and causes the magnetic grains and sleeve to slip on each other, thereby critically lowering the conveying ability of the sleeve and the amount of the developer scooped up. The resulting images suffer from granularity or halftone irregularity. On the other hand, if the restraining force of the magnetic member is short, then the magnetic grains fail to form the curtain and cannot promote the charging of the toner grains at all.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 62-138860, 5-6103, and 10-39620.
It is an object of the present invention to provide a developing device capable of stabilizing the amount of a developer to be scooped up, protecting the developer from deterioration and making the frictional charge of the developer adequate to thereby insure stable, high image quality, and an image forming apparatus using the same.
It is another object of the present invention to provide a developing device capable of making the magnetic restraining force of a magnetic member positioned on a metering member adequate, and an image forming apparatus using the same.
A developing device of the present invention includes a developer carrier for conveying a developer, which consists of toner and magnetic grains, deposited thereon to a developing position where the developer carrier faces an image carrier. A magnetic field forming member is fixed in place inside the developer carrier for exerting a normal magnetic force and a tangential magnetic force. A metering member faces the surface of the developer carrier with the intermediary of a gap for regulating the amount of the developer being conveyed by the developer carrier toward the developing position. Assume a normal flux density distribution range corresponding to, among normal flux densities Bmy of normal magnetic forces that can be measured on the surface of the developer carrier, a single maximum flux density max (Bmy). Then, the metering member is positioned such that its edge faces, within the above normal flux density distribution range, a portion of the developer carrier satisfying the following conditions (1) through (3):
(1) the normal flux density Bmy is at least one-half of the normal maximum flux density max (Bmy);
(2) the tangential flux density distribution range flux density Bmx, which can be measured on the surface of the developer carrier, located downstream of a portion of the developer carrier where the tangential flux density [[Bmy]] Bmx is maximum in the direction of developer conveyance; and
(3) the normal flux density distribution Bmy is equal to or higher than the tangential flux density Bmx.